The regulation of tyrosine aminotransferase mRNA by glucocorticoids and cyclic AMP is under investigation. Developmental studies using fetal liver explants in organ culture are focusing on the acquisition of the steroid hormone and cyclic AMP responsiveness and the initial appearance of tyrosine aminotransferase mRNA and enzyme activity immediately after birth. The phenomena of induction and superinduction of tyrosine aminotransferase mRNA using glucocorticoids or cyclic AMP and cycloheximide are being investigated with respect to the processes that regulate the turnover of this mRNA. The number of glucocorticoid and cyclic AMP-responsive mRNAs in rat liver is being surveyed to develop a model of the coordinacy of control of liver metabolism by these two complementary physiological regulators.